Thermal modeling and experimental validation of solar tunnel dryer: a clean energy option for drying surgical cotton

A battery of three walk-in-type solar tunnel dryers was commissioned at M/s Cotton Product of India, Udaipur (27′ 42°N, 75′ 33°E), for drying surgical cotton. Each dryer has a drying capacity of 600 kg of surgical cotton, with moisture content reduction averaging 40 to 5% (wb) in a single solar day. This investigation presents thermal modeling, energy and exergy analyses for a walk-in-type solar tunnel dryer. The predicted drying air temperature was 2–3°C higher than experimental values. The experimental and predicted values for energy efficiency of the drying process were found to vary from 1.051–1.793% and 1.298–2.224%, respectively, whereas those of exergy efficiency were found to vary from 0.039–0.072% and 0.030–0.058%, respectively. An economic analysis of the tunnel dryer under consideration has also been completed, and its greenhouse-gas mitigation potential has been calculated to better determine its usefulness as a replacement for light diesel oil (LDO) and liquefied petroleum gas (LPG) drying units. It was found that the payback periods of the solar tunnel dryer, when replacing LDO and LPG units, were 3.03 and 2.26 years, respectively whereas the benefit–cost ratio was 2.12 with respect to LDO units and 3.03 with respect to LPG units. Additionally, it was determined that a single unit of a tunnel dryer can reduce CO2 emissions by 12.15 tons per year when replacing an LDO unit, and by 6.72 tons per year when replacing an LPG unit.